1. Field of the Invention
The present invention relates to a method and apparatus for pre-heating combustion air and catalytically reducing noxious substances in flue gas.
2. Description of the Background
German Published Patent Application No. 3,335,917 discloses a pre-heater for air having a regenerative or recuperative configuration. In the prior art apparatus, the surfaces of the heat-exchanger elements on the flue gas side are provided with a coating which acts as a catalyst for NO.sub.x reduction. The regenerative air pre-heater shown in FIG. 5 of that publication includes a rotating drum and is equipped for radial gas influx. In this prior art apparatus, the heat-storage substance is composed of a plurality of individual movable elements in the form of spheres, e.g., hollow spheres or saddle-shaped bodies, which are provided with a catalyst coating. The drum is subdivided radially into individual chambers so as to accommodate the individual elements. This integration of the NO.sub.x reactor into the air pre-heater serves primarily to reduce the space requirement of the system. According to FIG. 6 of the reference, a first air pre-heater with the shape of a tube or plate and a second air pre-heater are connected in series in the manner of a heat-exchange column. The heat-exchange column includes a storage substance composed of a plurality of separately movable elements which are turned by a transporting device. However, only the heat-exchange elements of the plate- or tube-shaped heat-exchanger are provided with a catalyst coating. In both cases, the elements coated with the catalyst layer form static or quasi-static components. Due to their decreasing catalytic activity over their service life, these elements must be considerably larger than would be necessary in theory to attain a service life which is economically justifiable for the operation of a power plant, e.g., up to three years. For example, a catalyst subdivided into three layers will have an average service life of only about 1.3 years per layer. This is the case even if it is coated with a substance roughly 35% above the theoretical requirement. In addition, the entire system must be shut down after reaching its lower activity limit for the exchange or partial exchange of the catalyst-coated elements.
It is known to perform flue gas desulfurization before catalytic reduction of the noxious substances as disclosed in German Published Application No. 3,407,277. This substantially protects the sensitive catalyst from sulfur compounds. To keep the required external energy for reheating the flue gas as low as possible, however, a regenerative gas preheater is connected in the circuit. Leakages from this regenerative gas preheater are known to adversely affect the efficiency of the catalyst and, thus, to require a corresponding enlargement of the quantity of catalyst, because this preheater effects polluting the cleaned flue gas with uncleaned flue gas.